Among the most widely-used instruments for measuring surface topographies are interferometers, which use the wave nature of light to map variations in surface height with a high degree of accuracy. It is generally accepted in the art that the most accurate interferometers in common use today are based on the principle of phase shifting. Modern phase-shifting interferometers are typically comprised of an optical system, an electronic imaging system, a computer-based or otherwise automated controller, and means for introducing a reference phase shift. Phase shifting interferometry ("PSI") is, for example, described in detail in Chapter 14 of the book Optical Shop Testing, edited by Daniel Malacara (Wiley, New York, 1992). Briefly described, PSI typically involves the electronic storage of intensity patterns measured for a sequence of three to five reference phase shifts. These stored intensity patterns are then analyzed, as by a computer-based digital signal processor, to recover the original wavefront phase through analysis of the variations of intensity as a function of phase shift. When such PSI-based instruments are properly adjusted, they are capable of measuring surface topography with a resolution on the order of one-thousandth the wavelength of light.
Certain aspects of all prior an PSI instruments are critical to obtaining topographical surface measurements with the intended high degree of precision. These include precise adjustment of the reference phase and incremental phase changes during operation of the interferometer, and substantially complete isolation of the interferometer from vibration, most especially the low-frequency vibrations typical of most production environments.
Currently the most common method and arrangement for introducing the reference-phase shifts required for PSI is by mechanical translation or movement of the reference surface. The total amount of mechanical motion necessary for this purpose is normally less than the wavelength of light, and is often effected by controlled operation of a piezo-electric transducer (PZT), or equivalent assembly, which must be carefully (and often repeatedly) calibrated to provide suitably accurate and repeatable phase shifts. An alternative methodology known in the art involves selectively tuning the wavelength of the source light for the interferometer in such a way as to effect small phase shifts. This alternative method also requires careful calibration, and may even necessitate continuous monitoring of the source wavelength.
It is well known that distortions of the phase shift typically result in unacceptable measurement errors in prior art PSI instruments. Thus, significant errors can result when the reference phase shift is nonlinear or is not properly calibrated. In an article entitled Linear Approximation For Measurement Errors In Phase Shifting Interferometry, by J. van Wingerden, H. H. Frankena and C. Smorenburg, 30 Applied Optics 2718-29 (1991), these errors are described in some detail, including their form and magnitude. Several other articles, such as that by K. Kinnstatter, Q. W. Lohmann, J. Schwider and N. Streibl entitled Accuracy Of Phase Shifting Interferometry, 27 Applied Optics 5082-89 (1988), note the importance of careful calibration and suggest specific arrangements for measuring the magnitude of phase shifts. Deviations from linear motion are particularly troublesome in prior art PSI instruments, since even relatively small nonlinearities can lead to unacceptably large errors in the resulting surface height calculations. As a consequence, expensive and cumbersome high-voltage PZT assemblies must be employed to assure that the phase shift is as linear as possible, and time-consuming procedures must be performed to attain substantially perfect calibration of phase shifts. Problems associated with calibrating and linearizing wavelength shifts have also effectively precluded the widespread use of wavelength-tunable laser diodes in interferometers as an alternative to mechanical displacement of interferometer elements.
Low-frequency and other vibrations occurring during the interferometer data acquisition cycle also significantly distort the phase shifts, thus resulting in additional errors which in prior art instruments can only be avoided through careful elimination of all sources of vibration. Commonly-employed methods of vibration isolation involve the use of large granite or air-suspension tables, in conjunction with a heavy framework for supporting the interferometer optics. In addition, the measurement system must often be physically separated from production and assembly areas, resulting in significantly higher costs to manufacturing industries that use interferometers to verify the surface quality of manufactured articles.
There is therefore an unmet need for an apparatus and method for accurately measuring surface topography without dependence upon the high-precision phase shifting mechanisms and procedures and expensive vibration isolation arrangements of the prior art.